Import collapse_rfc2231_value for the compatibility module.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/sources/models.py

@@ -1,7 +1,6 @@
 from __future__ import unicode_literals
 
 from email import message_from_string
-from email.Utils import collapse_rfc2231_value
 from email.header import decode_header
 import imaplib
 import json
@@ -28,6 +27,7 @@ from django.utils.translation import ugettext_lazy as _
 
 from model_utils.managers import InheritanceManager
 
+from common.compat import collapse_rfc2231_value
 from common.compressed_files import CompressedFile, NotACompressedFile
 from common.utils import TemporaryFile
 from converter.literals import DIMENSION_SEPARATOR